Watch pom-pom crabs fight over tiny anemones, which they hold like boxing gloves

"Pom-pom" or "boxer" crabs hold sea anemones in their claws and will duke it out with other crabs to protect their anemones or gain a new one, new research shows. The crabs then induce the anemones to reproduce asexually by "splitting" them with their claws to create two. (Yisrael Schnytzer​​, Yaniv Giman​, Ilan Karplus, Yair Achituv)

Crabs are a diverse bunch of crustaceans. Some, like the pea crab, are parasites so small that they spend their lives hiding inside oyster shells. Others are huge. The largest of Japanese spider crabs, tipping the scales at up to 40 pounds, weigh about as much as basset hounds. What makes the pom-pom crab — also known as the boxer crab — so special is not its size, but what it grips in a unique set of claws. Where many crabs have crushing pincers for defense or attack, the pincers of the pom-pom crab are shaped like forceps, with which it always holds a pair of stinging sea anemones.

The effect, like the name pom-pom crab suggests, is of a cheerleader holding a small frizzy ball, or perhaps a young teen thrusting a carnation at a homecoming date. The crabs use the sea anemones like nets, to catch food; the crabs also use the anemones to ward off larger animals that would prey on the crabs. The sea anemones, for their efforts, get fed scraps.

But it is still a mystery where, exactly, the crabs acquire this particular species of anemone in the wild. The anemones are separate animals. The crabs, which are born as free-floating larva and only later molt into adults with claws, must somehow catch them. “Every single crab we’ve ever found was found holding an anemone, but we’ve never found a free-living anemone,” marine biologist Yisrael Schnytzer, currently of Maine’s Woods Hole Laboratory, told New Scientist. “We started making these crazy theories, like maybe there’s a secret anemone garden.”

A report published Jan. 31 in the journal PeerJ has not, sadly, found a secret club of crab gardeners. Schnytzer and his colleagues at Bar-Ilan University, Israel, instead found that if crabs lacked two full anemone bouquets, the crabs would force the tentacled creatures to divide into clones.

The scientists caught a few dozen crabs and removed some of their anemones; wild crabs are rarely if ever found without the symbiotic poms in claw. “Laboratory observations showed that the removal of one anemone from a crab induces a ‘splitting’ behavior, whereby the crab tears the remaining anemone into two similar parts, resulting in a complete anemone in each claw after regeneration,” the biologists wrote. In other words, a crab with only one pincer’s worth of anemone would pull it apart, and the anemone halves would clone themselves back into twin poms.

Something similar happened after the scientists removed the anemones completely from one crab, and then put it in a cage with a fully endowed partner. The crabs fought — not causing mortal damage but “almost always leading to the ‘theft’ of a complete anemone or anemone fragment by the crab without them,” wrote the researchers. “Following this, crabs ‘split’ their lone anemone into two.” DNA analysis of the anemones in the wild confirmed that the wild poms were genetic clones, indicating that similar splits may occur in the ocean.

The pom-pom crab, the scientists speculated, is perhaps the only animal on the planet that controls another species’ growth, feeding and asexual reproduction.